The present invention relates to a metallic sheet hydroforming method using metallic sheets as blanks, as well as a forming die used in the method and a formed part on workpiece.
A sheet hydroforming method is known in which peripheral portions of two metallic sheets (hereinafter referred to also as xe2x80x9cblanksxe2x80x9d) are bonded together, then a fluid is introduced between the blanks, followed by the application of pressure of the fluid, causing the blanks to be bulged.
FIGS. 1A, 1B, 1C, and 1D illustrate a forming method described in Japanese Patent Application Laid Open No. 47-033864. FIG. 1A is a perspective view of two blanks which are each in a ring shape, FIG. 1B is a sectional view of a die portion before a forming work in which two blanks bonded together at their peripheral portions are set between upper and lower dies, FIG. 1C is a sectional view of the die portion in a completed state of sheet hydroforming, and FIG. 1D is a perspective view of a bent tubular part obtained by cutting a formed part on workpiece crosswise.
The blanks shown in FIG. 1A are in a state before being subjected to peripheral bonding into a single blank. The blanks are two ring-like blanks 100 and 102. A pipe-like nozzle 101 is bonded, for example by welding, to the position of a thru-hole formed in a planar portion of the blank 100. The blanks 100 and 102 are put one on the other and are bonded together for example by welding throughout the whole inner and outer peripheries thereof to afford a workpiece (xe2x80x9cbonded blankxe2x80x9d hereinafter).
First, as shown in FIG. 1B, the bonded blank, indicated at 103, is set on a lower die 104, then an upper die 105 is brought down from above by means of a drive unit (not shown), an outer peripheral portion 103a and an inner peripheral portion 103b of the bonded blank are pressed and sandwiched in between the upper and lower dies, and the nozzle and a pipe 106 are connected together through a thru-hole 105b formed in the upper die. Die cavities 104a and 105a having an inner contour shape which is the same as an outer contour shape of product are formed in the lower die 104 and upper die 105, respectively. Then, a fluid is introduced between mating surfaces of the bonded blank from a pump (not shown) through the pipe and nozzle, followed by the application of pressure, causing the bonded blank to bulge.
The full-circled bonding of the blanks 100 and 102 is for the purpose of preventing the leakage of fluid from the mating surfaces of the bonded blank.
As shown in FIG. 1C, by raising the pressure of the fluid 107, the metallic sheets bulge into contact with inner walls of the die cavities 104a and 105a and the forming work is completed. Thereafter, the internal fluid pressure is decreased, the pipe is pulled out, the upper die is raised, a ring-like hollow shell 108 is taken out, and the interior fluid is discharged from the nozzle. The formed part on workpiece is cut crosswise into a desired product size, affording a bent tubular part 109.
The above method brings about the following advantages in comparison with a method wherein upper and lower parts are manufactured separately by a press stamping method for example and thereafter both are bonded and assembled together by, say, welding.
The first advantage is that the bonding is easy because the blanks are bonded in a flat state. In case of bonding upper and lower stamped parts, it is necessary to use a jig for shape correction and alignment with respect to each of elastically recovered stamped parts, and the number of working steps increases.
The second advantage is that since the working is done using upper and lower dies and fluid, the tool expenses are low in comparison with the press stamping method.
The third advantage is that since a stretch formed portion is created by forming with a tensile stress based on a fluid pressure, a problem such as body wrinkling, which is often observed in press stamping, is difficult to occur.
These advantages are also true of the following prior art examples.
FIGS. 2A and 2B are diagrams for explaining a forming method disclosed in Japanese Patent Application Laid Open No. 63-295029. FIG. 2A is a perspective view of a bonded blank before forming and FIG. 2B is a perspective view of a formed part on workpiece.
In this method, as shown in FIG. 2A, two blanks 110 and 111, which are fabricated in a developed shape of a desired product by a press punching method for example, are put one on the other and outer peripheral edges 112 of their mating surfaces are bonded together by a laser welding method for example to afford a bonded blank 113. The bonded blank 113 is then set within upper and lower dies and pressurized fluid is introduced between the mating surfaces from a suitable bonded blank opening, causing the blank to bulge. As shown in FIG. 2B, the resulting formed part is an engine manifold part 117 having a welded line 116, in which manifold portions 114 and a trunk portion 115 are cut at their end portions.
FIGS. 3A, 3B, 3C, 3D, and 3E are diagrams explanatory of a forming method disclosed in Japanese Patent Application Laid Open No. 09-029329. FIG. 3A shows blanks 120 and 121 before bonding, the blanks 120 and 121 being formed with half conical recesses 120a and 121a on flange, respectively, by press stamping. FIG. 3B shows a bonded blank 123 obtained by superimposing blanks 120 and 121 one on the other and bonding the two by, say, laser welding along a continuous welded line 123b except a conical inlet 123a. FIG. 3C shows a state in which a peripheral portion of the bonded blank 123 is held grippingly by lower die 125 and upper die 126 attached to a press machine (not shown), then a conical head 127b of an injection nozzle 127 is inserted into the inlet 123 by means of a drive unit (not shown) and is pushed against half conical recesses 125b and 126b on die surfaces. Then, pressurized fluid is injected between the blank mating surfaces from a pump (not shown) through an intra-nozzle channel 127a, causing die cavities 125a and 126a having the same inner contour shape as an outer contour shape of product to bulge. With this bulging motion, a flange 123c which has been held grippingly by the dies 125 and 126 moves gradually toward the die cavities 125a and 126a except the portion near the inlet. FIG. 3D shows a completely bulged state in which the blanks were brought into contact with inner walls of the die cavities 125a and 126a by increasing the pressure of fluid 128. Thereafter, the pressure of the fluid is decreased and the fluid is discharged from the inlet 123a to afford a formed part 129. FIG. 3E shows an example of a tubular part 129 obtained by cutting off the portion located outside the welded line 123b and also cutting off both ends of the stretch formed portion of workpiece.
In the above sheet hydroforming methods, the following problems are encountered in injecting the pressurized fluid between the mating surfaces of blanks.
In the forming method shown in FIGS. 1A, 1B, 1C, and 1D it is necessary that the nozzle be bonded to the associated blank while assuming a position which permits smooth insertion of the nozzle into the thru-hole formed in the upper die as the bulging motion proceeds. This requirement may not be satisfied in some particular sectional shape of product. Besides, since connection and disconnection between the nozzle and the pipe are troublesome, the productivity is low and automation is difficult.
In the forming method disclosed in Japanese Patent Application Laid Open No. 63-295029, which is illustrated in FIGS. 2A and 2B, there is made no reference to a pressurized fluid injecting method.
In the forming method illustrated in FIGS. 3A, 3B, 3C, 3D, and 3E there arises a problem of how to seal the pressurized fluid between the bonded blank inlet and the conical portion of the nozzle.
FIG. 4 is a front view showing the inlet 123a as seen in the direction of arrow A in FIG. 3B. Since bent portions 130 are rounded at a radius at least equal to the blank thickness, there are formed tapered grooves 131 and hence it is necessary to prevent the leakage of pressurized fluid from the grooves 131. But in Japanese Patent Application Laid Open No. 09-029329 there is found no explanation about a method to be taken for the prevention of such fluid leakage.
As noted above, as to the sheet hydroforming in which a pressurized fluid in injected between the mating surfaces of the bonded blank, working methods are disclosed in the prior art references, but a concrete pressurized fluid injecting method superior in utility is not disclosed therein.
A description will now be given about dent resistance. As to shallow-bottom panel parts (also referred to simply as xe2x80x9cpanel partsxe2x80x9d hereinafter) formed by metallic sheets, typical of which are automobile door panel, bonnet, and trunk lid, it is required for them to possess a property such that a dent is difficult to remain after the application of a local external force to the panel surface, i.e., dent resistance. For example, in the case of the automobile door panel, if a dent defect (xe2x80x9cdentxe2x80x9d hereinafter) occurs due to pressing with a thumb near a door handle at the time of opening or closing of the door concerned, the appearance of the door is impaired.
Also in the case of the automobile bonnet and trunk lid, their appearance is impaired by the dent caused by pressing with palms when they are closed. Not only the pressing with fingers and palms, but also the collision of a flying stone with a panel part during vehicular running may form a dent. Dent resistance is a subject to be attained not only in such vehicular panel parts as mentioned above but also in panel parts of home electric appliances such as the refrigerator door.
FIGS. 5A, 5B, and 5C show an example of a method for evaluating quantitatively how dent is difficult to occur, i.e., dent resistance. FIG. 5A is a sectional view showing a state in which a load P is imposed on a panel surface 200 of a panel part 201 through an indentor 150 having a semispherical tip. FIG. 5B shows a load-removed state, in which such a dent 151 of depth d as shown in FIG. 5C is formed in a loaded portion B.
The larger a critical load P of inducing a dent of depth d (e.g., 0.02 mm) which poses a problem as product, the higher the dent resistance. The critical load P is designated a dent resistance load. It goes without saying that the dent resistance load should be measured at unified test conditions because the dent resistance load is influenced by the radius of curvature of the indentor tip or by the hardness of the indentor in case of the indentor being an elastic indentor.
Further, dent resistance is influenced by the thickness of a panel part and the yield strength of the material used. Dent resistance becomes lower with a decrease of the panel thickness and yield strength. Therefore, for reducing the panel part thickness to reduce the weight of the panel part, it is necessary to increase the strength of the panel surface so as to prevent deterioration of the dent resistance.
FIG. 6 illustrates a method of sampling a tensile specimen from a panel surface. The aforesaid yield strength indicates a yield strength determined using a tensile specimen 202 cut out from a portion of the panel part 201 which portion involves the problem of dent resistance, as shown in FIG. 6.
FIG. 7 schematically illustrates a relation between a stretch strain (e) and a tensile stress ("sgr") (tensile load/original sectional area of specimen) in a tension test for a sheet blank and also in a tension test (xe2x80x9cpanel tension testxe2x80x9d hereinafter) using the specimen sampled from the panel part, i.e., a stress-strain diagram.
In the same figure, a curve OAB represents the result of the blank tension test, in which the point A is a yield point, while a curve Oxe2x80x2Axe2x80x2B is a stress-strain diagram in the panel tension test, with point Axe2x80x2 being a yield point. A clear difference between the two curves is a difference between the stress at point A and the stress at point Axe2x80x2. A yield point stress ("sgr"Axe2x80x2) (xe2x80x9cpanel surface yield point stressxe2x80x9d hereinafter) in the panel tension test is larger than a yield point stress ("sgr"A) (xe2x80x9cblank yield point stressxe2x80x9d hereinafter) in the blank tension test. This is due to the influence of work hardening caused by the imposition of a permanent strain on point Oxe2x80x2 in the panel manufacture.
Since a dent which causes a problem in the appearance beauty is formed by very small plastic deformation of a panel part under the action of a local external force, it is presumed that the larger the panel surface yield point stress ("sgr"Axe2x80x2), the more improved the dent resistance.
The panel parts referred to previously have heretofore been manufactured by press stamping of sheet metal.
FIGS. 8A, 8B, and 8C illustrate tools used in press stamping, a state of stamping, and an example of a formed part. FIG. 8A illustrates a state in which a blank 203 is set on a die 204 fixed to a press bed 211 and a peripheral portion 203b of the blank is binded against a die surface 204a at a predetermined load with use of a blank holder 205, the blank holder 205 being attached to outer slide 212 which has been moved down from above by means of a drive unit (not shown).
At this time, the peripheral portion of the blank is clamped with concave and convex portions 208 (xe2x80x9cbeadsxe2x80x9d hereinafter) formed opposedly on both die surface 204a and blank holder surface 205b around a die cavity 204e. Next, a punch 206 attached to inner slide 213 which has been brought down from above by another drive unit (not shown) is moved down through a space formed inside the blank holder. When the punch 206 comes into contact with a sheet blank 203a positioned within a die cavity, a tensile force acts on the blank because the peripheral portion of the blank is pressed by both die and blank holder.
With descent of the punch, the said tensile force increases and the peripheral portion of the blank is pulled in toward the die cavity.
FIG. 8B shows a state in which the punch has descended to a bottom of the die cavity and a stretch formed portion (also referred to as xe2x80x9cpanel surfacexe2x80x9d) 207a is formed between a punch surface 206a and a die bottom 204b. Thereafter, the punch and subsequently the blank holder are raised and a formed part 207 is taken out.
FIG. 8C illustrates the formed part. Bead patterns 207d formed by the beads 208 remain on a peripheral portion (xe2x80x9cflangexe2x80x9d hereinafter) 207b of the formed part. In steps which follow the flange is cut off to obtain the panel part 201.
In the above press stamping it is important that the stretch formed portion, or the panel surface, be allowed to undergo a stretch deformation with a tensile force.
The first reason is that in case of the panel surface being a curved surface and if stretch deformation is extremely small, the product is prevented from having a predetermined radius of curvature due to an elastic recovery. In this case there also arises an inconvenience such that a elastic stiffness (difficulty of elastic deflection) of the panel surface is low and there occurs xe2x80x9ccanningxe2x80x9d when a local load is applied to the panel surface.
The second reason is that if an increase in yield stress ("sgr"Axe2x80x2) of the panel surface induced by stretch deformation is small, the foregoing dent resistance becomes insufficient.
The material of the panel surface is in a biaxially stretched state under the action of a surrounding tensile force, and for increasing the amount of stretch deformation of the panel surface it is necessary to increase the tensile force acting on the panel surface during press forming. The larger the strength and thickness of the metallic sheet and the area of the panel surface are, the larger the tensile force required for stretching the panel surface is. This tensile force is created by resistance (xe2x80x9cdrawing resistancexe2x80x9d hereinafter) which is induced when the flange is pulled into the die cavity by the punch. The larger the holding force (also referred to as xe2x80x9cblank holder forcexe2x80x9d hereinafter) of the blank holder and the larger the flange area, the higher the drawing resistance.
However, the blank holder force is restricted by the capacity of the press machine used and the flange area is set to a minimum area from the standpoint of blank yield, so with these means it is difficult to ensure a required drawing resistance. The bead compensates for the deficiency in the drawing resistance. A drawing resistance is created by a bending deformation induced when the flange passes the bead. Usually, the bead is arranged at a position where the drawing resistance of the flange is small, such as a straight side portion of the die cavity contour, as shown in FIG. 8C.
In press stamping, a problem is encountered such that the drawing resistance is difficult to be transmitted directly as a force of deforming the panel surface. The following two are considered as factors of this problem.
According to the first factor, a friction occurs between the punch surface and a punch shoulder 206b and this frictional force suppresses the stretch deformation of the panel surface. The larger the area of the punch surface is, the more influential the friction is.
The second factor is a bending at the punch shoulder. For the material to stretch at the panel surface it is necessary that the material moves to the side wall through the punch shoulder. This is obstructed by both bend and friction at the punch shoulder. The smaller the profile radius of the punch shoulder is, the greater the influence thereof is.
Since the stretch deformation of the panel surface is suppressed by the above factors, it is difficult to increase the stretch deformation of the panel surface even if a forming depth (H) shown in FIG. 8C is increased. A value (xe2x80x9cequivalent strain of the stretched formed portionxe2x80x9d or xe2x80x9cxcex5 eqxe2x80x9d hereinafter) obtained by converting a biaxial tensile elongation on the panel surface by press stamping into a uniaxial tensile elongation is 2% or so at most and thus the deficiency in dent resistance becomes a problem even if the elastic stiffness is satisfied.
Further increasing the equivalent strain of the stretch formed portion and improving the yield stress ("sgr"Axe2x80x2) of the panel surface by work hardening is difficult with the above press stamping method and there has been adopted the thinking that a strength characteristic of a metallic sheet blank is to be selected so as to satisfy a panel surface yield stress ("sgr"Axe2x80x2) required for dent resistance even if xcex5 eq is small. That is, in case of decreasing the thickness of a panel part for the reduction of weight, which brings a decrease in dent resistance, it is necessary to change to a metallic sheet of a higher strength so as not to cause a lowering of dent resistance. For example, what is called a high strength steel sheet has so far been used.
As the yield point stress of blank increases, an elastic recovery after press forming becomes larger, thus giving rise to the problem that a predetermined product shape cannot be obtained. Thus, an upper limit is encountered in the yield point stress ("sgr"A) of blank. Generally there is used a blank having a yield point stress of 280 Mpa or less.
As noted above, since xcex5 eq obtained in press stamping is 2% or so at most, the panel surface yield point stress ("sgr"Axe2x80x2) is 320 MPa or so at most. Therefore, it is inevitably required to select a suitable sheet blank thickness so as to satisfy a required dent resistance at such a panel surface yield point stress, and thus a limit is encountered in reducing the thickness and weight of a panel part.
The present invention has been accomplished in view of the above-mentioned problems and it is an object of the invention to provide a sheet hydroforming method wherein a pressurized fluid can be injected between mating surfaces of two blanks easily and without leakage of the fluid, further provide a forming die used therein and a formed part on workpiece obtained by the method, as well as the above method able to improve dent resistance, a forming die used therein and a formed product obtained by the method.
For achieving the above-mentioned object, the inventors in the present case have studied the foregoing conventional problems and obtained the following knowledge.
a) A thru-hole to introduce a pressurized fluid, which leads to a holding surface of a die, is formed in the die, and a pierced hole to introduce the fluid formed in a portion of stacked metallic sheets, which portion is in contact with the holding surface of the die, is positioned with the thru-hole formed in the die, then the pressurized fluid is injected between mating surfaces of the metallic sheets from the thru-hole in the die through the pierced hole on blank, allowing a channel to be formed to introduce the pressurized fluid into a portion to be bulged. According to this method, the fluid can be injected between the mating surfaces of the metallic sheets easily without leakage thereof, whereby the forming work can be done efficiently.
b) A dent load of a formed part increases with an increase in equivalent strain of the stretch formed portion of workpiece, but when the equivalent strain of the stretch formed portion (also called xe2x80x9cequivalent strain of the panel surfacexe2x80x9d hereinafter) saturates at 10% or so and increases to a further extent, the dent resistance load becomes lower. This is because a lowering in dent resistance caused by a decrease in thickness of stretch formed portion becomes more influential than the improvement in dent resistance of the stretch formed portion of workpiece based on work hardening.
The present invention has been accomplished on the basis of the above knowledge and the gist thereof is summarized in the following points (1) to (10):
(1) A metallic sheet hydroforming method comprising:
pressing and clamping two stacked metallic sheets between holding surfaces of a pair of upper and lower dies having die cavities of the same inner contour shape as an outer contour shape of product;
forming a thru-hole in one of the dies for the injection of a fluid, the thru-hole being led to the holding surface of the one die;
positioning a pierced hole for the injection of the fluid with the thru-hole in the one die, the pierced hole being formed in a portion of one of the metallic sheets which portion is in contact with the holding surface of the one die; and
introducing the fluid in a pressurized state between the mating surfaces of the two stacked metallic sheets from the thru-hole in the one die through the pierced hole formed in the one metallic sheet blank, thereby causing the metallic sheets to bulge within a space defined by the die cavities.
(2) A metallic sheet hydroforming method as described in the above (1), wherein the two stacked metallic sheets are bonded together at respective mating surfaces in an area outside to-be-bulged portions and outside the thru-hole formed in one metallic sheet.
(3) A metallic sheet hydroforming method as described in the above (1) or (2), wherein after the metallic sheets have been bulged by introducing the pressurized fluid between the mating surfaces of the metallic sheets, portions which are in contact with the holding surfaces of the dies and which are unnecessary as product are cut off, thereby obtaining two formed parts at a time.
(4) A metallic sheet hydroforming method as described in any of the above (1) to (3), wherein the portion(s) to be bulged of one or both of the metallic sheets is (are) formed in a three-dimensional shape beforehand.
(5) A metallic sheet hydroforming method as described in any of the above (1) to (4), wherein after the metallic sheets have been stretch formed, one or both stretch formed portion(s) of workpiece is (are) punched to form a hole(s) with a punch incorporated in one or both of the dies, and the fluid is discharged from the hole(s).
(6) A metallic sheet hydroforming method as described in any of the above (1) to (5), wherein an equivalent strain of the stretch formed portion of workpiece obtained by bulging the metallic sheets is in the range of 2% to 10%.
(7) A hydroforming die comprising:
a pair of upper and lower dies having die cavities of the same inner contour shape as an outer contour shape of a product;
a thru-hole formed in one of the dies for the injection of a pressurized fluid, the thru-hole being led to a holding surface of the one die; and
a channel-forming groove formed in a holding surface of the other die, the channel-forming groove being extended to the die cavities through a portion opposed to the thru-hole formed in the one die.
(8) A hydroforming die as described in the above (7), wherein one or both of the dies has (have) means for piercing a fluid discharge hole on a stretch formed portion on workpiece after forming.
(9) A hydroformed product obtained by injecting a fluid between mating surfaces of two stacked metallic sheet blanks and pressurizing the fluid to bulge the blanks, the hydroformed product having a convex fluid channel extending to a stretch formed portion and also having a pierced hole on the blank opposed to the convex fluid channel.
(10) A hydroformed product obtained by injecting a fluid between mating surfaces of two stacked metallic sheets and pressurizing the fluid to bulge the blanks, the product having an equivalent strain of the stretch formed portion of workpiece in the range of 2% to 10%.
The two stacked metallic sheets are obtained by superimposing one metallic sheet on the other metallic sheet. As one or both of such blanks there are included a laminate of plural metallic sheets and a composite of both a metallic sheet and a sheet of a non-metallic material such as plastic.